This invention relates to welding apparatus and more particularly to welding apparatus for welding a sleeve within a tube using a laser beam.
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak occurs therein which allows the fluids to mingle. When this occurs, it is sometimes necessary to either plug the tube so that the fluid does not flow through the tube or repair the tube, thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube-type heat exchangers are commonly referred to as steam generators. When a defect occurs in a tube of a nuclear steam generator that allows the coolant in the tube to mingle with the coolant outside of the tube, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of the nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube must either be plugged or repaired so that the coolant does not leak from the tube. This prevents contamination of the fluid surrounding the tubes.
There are several methods known in the art for repairing heat exchange tubes; however, many of these methods are not applicable to repair of heat exchange tubes wherein the tube is not readily accessible. For example, in a nuclear steam generator the physical inaccessibility of defective heat exchange tubes and the radioactive nature of the environment surrounding the heat exchange tubes present unique problems to repairing heat exchange tubes that do not normally exist in other heat exchangers. For these reasons, special methods have been developed for repairing heat exchange tubes in nuclear steam generators. Typically, the method used to repair a heat exchange tube in a nuclear steam generator is one in which a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube is inserted into the defective tube and attached to the defective tube to bridge the defective area of the tube. This type of repair method is generally referred to as "sleeving". Previous sleeving development work has been concerned with obtaining a relatively leakproof joint between the sleeve and the tube by brazing, arc welding, explosive welding, or other joining means. Due to the need for cleanliness, close fittings, heat applications, and atmospheric control, these metallurgical bonding techniques have problems which are not easily solvable in areas such as a nuclear steam generator where human access is limited.
In the braze sleeving methods such as the one described in U.S. patent application Ser. No. 790,010, filed Oct. 22, 1985, in the name of W. W. Cheng and entitled "Improved, Continuous Movement Brazing Process", which is assigned to the Westinghouse Electric Corporation, it is necessary to heat a braze metal to its melting point in order to form the braze bond between the sleeve and tube. One way to heat the braze material is by inserting a heating apparatus, such as the one described in U.S. patent application Ser. No. 720,107, filed Apr. 4, 1985 in the name of W. E. Pirl et al. and entitled "Improved Braze Heater Assembly and Method", which is assigned to the Westinghouse Electric Corporation, inside the sleeve within the tube. This process utilizes specially made sleeves provided with a recess containing braze material, careful expansion of the portion of the sleeve containing the braze material into contact with the tube wall, and precise positioning of the heating apparatus within the sleeve at the site of the braze material, all of which are hampered by the inaccessibility of the work area.
Welding methods for internally welding sleeves to tubes in heat exchangers require specially designed welding equipment. One such apparatus is described in U.S. Pat. No. 4,510,372, which issued Apr. 9, 1985, in the name of R. M. Kobuck et al. and is entitled "Sleeve-To-Tube Welder", which is assigned to the Westinghouse Electric Corporation. With such a device, care must be taken to avoid arc piercing the sleeve and tube.
Laser welding is an attractive alternative to arc welding and brazing for joining metals since it is faster and produces a smaller heat affected zone. However, spatial constraints have heretofore prevented the use of laser welding for sleeve-to-tube welding of nuclear steam generator tubes due to the bulkiness of lasers capable of delivering sufficient power for welding. The invention described herein presents a method and apparatus for such welding.